Polyphase electrical systems are used extensively in many types of industries, and three phase systems have become commonplace. There are many advantages to the use of polyphase electrical equipment, including the elimination of pulsation inherently present in single phase electrical operation. Large machines operating on single phase may pulsate objectionably, leading to excessive vibration. Polyphase current provides a constant power level if the phases are properly balanced. Polyphase operation minimizes eddy currents in field windings of electrical apparatus and therefore reduces head build-up in the apparatus, and the polyphase machines operate more efficiently and economically, in that for a given frame size, the output of a polyphase machine is greater than the output of a single phase machine.
For these and other reasons, many large manufacturing machines, television and radio transmitters, and computers are often operated on three phase current, which is usually supplied from the electrical utility serving the establishment. In some situations, the cost of extending three phase current to the location of the equipment may not justify the savings obtained in operating the three phase current. Further, if an outage occurs in one or two of the supply lines of a three phase source, serious complications can arise. For example, some types of knitting and weaving machines used in the fabric industry operate on three phase current. If a phase is lost, or if fluctuations in the current are present, the machines may either stop or slow down, resulting in desynchronization of operation. When this occurs, the machine jams, and though a power outage may be only for an instant, the time required to clear and restart the machine can result in substantial downtime of the machine. For television and radio transmitters, a phase outage of several seconds may result in a broadcast outage of several minutes, and in the operation of computers, a phase outage can result in the clearing of an established short-time memory in the computer. These and other types of problems can occur when a phase outage of seconds occurs, and if the phase outage persists, motor burnout and other serious mechanical damage can result.
Devices have been produced which will provide power for a given number of cycles if one or more phases are lost; however, these devices are only a temporary back-up, and have been used primarily for computers so that a short phase loss of perhaps 50 to 500 cycles will not cause an erasure of the temporary memory of the computer. Other types of back-up and/or warning systems have been used in which the phases are monitored and a relay or other means is provided for each, so that if a phase drops, only a minor interruption occurs. Warning devices are available which by alarm or light indicate the loss of a phase so that equipment can be shut down before damage thereto occurs; however, none of the previous devices could provide an extended back-up to continue a three phase current supply uninterrupted if one or two phases experienced outages.
In the operation of various systems of the foregoing types, other transient malfunctions sometimes occur which in some instances have only minor effect on the operation or performance, but which in other instances may cause serious problems or result in unsatisfactory operation of the system. In the electrical supply for the system, spikes and other transients may occur which prevent the equipment or systems from operating at maximum efficiency or providing optimum performance.